linux/arch/x86_64/kernel/tsc_sync.c
john stultz 5a90cf205c [PATCH] x86: Log reason why TSC was marked unstable
Change mark_tsc_unstable() so it takes a string argument, which holds the
reason the TSC was marked unstable.

This is then displayed the first time mark_tsc_unstable is called.

This should help us better debug why the TSC was marked unstable on certain
systems and allow us to make sure we're not being overly paranoid when
throwing out this troublesome clocksource.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
2007-05-02 19:27:08 +02:00

188 lines
4.2 KiB
C

/*
* arch/x86_64/kernel/tsc_sync.c: check TSC synchronization.
*
* Copyright (C) 2006, Red Hat, Inc., Ingo Molnar
*
* We check whether all boot CPUs have their TSC's synchronized,
* print a warning if not and turn off the TSC clock-source.
*
* The warp-check is point-to-point between two CPUs, the CPU
* initiating the bootup is the 'source CPU', the freshly booting
* CPU is the 'target CPU'.
*
* Only two CPUs may participate - they can enter in any order.
* ( The serial nature of the boot logic and the CPU hotplug lock
* protects against more than 2 CPUs entering this code. )
*/
#include <linux/spinlock.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/smp.h>
#include <linux/nmi.h>
#include <asm/tsc.h>
/*
* Entry/exit counters that make sure that both CPUs
* run the measurement code at once:
*/
static __cpuinitdata atomic_t start_count;
static __cpuinitdata atomic_t stop_count;
/*
* We use a raw spinlock in this exceptional case, because
* we want to have the fastest, inlined, non-debug version
* of a critical section, to be able to prove TSC time-warps:
*/
static __cpuinitdata raw_spinlock_t sync_lock = __RAW_SPIN_LOCK_UNLOCKED;
static __cpuinitdata cycles_t last_tsc;
static __cpuinitdata cycles_t max_warp;
static __cpuinitdata int nr_warps;
/*
* TSC-warp measurement loop running on both CPUs:
*/
static __cpuinit void check_tsc_warp(void)
{
cycles_t start, now, prev, end;
int i;
start = get_cycles_sync();
/*
* The measurement runs for 20 msecs:
*/
end = start + tsc_khz * 20ULL;
now = start;
for (i = 0; ; i++) {
/*
* We take the global lock, measure TSC, save the
* previous TSC that was measured (possibly on
* another CPU) and update the previous TSC timestamp.
*/
__raw_spin_lock(&sync_lock);
prev = last_tsc;
now = get_cycles_sync();
last_tsc = now;
__raw_spin_unlock(&sync_lock);
/*
* Be nice every now and then (and also check whether
* measurement is done [we also insert a 100 million
* loops safety exit, so we dont lock up in case the
* TSC readout is totally broken]):
*/
if (unlikely(!(i & 7))) {
if (now > end || i > 100000000)
break;
cpu_relax();
touch_nmi_watchdog();
}
/*
* Outside the critical section we can now see whether
* we saw a time-warp of the TSC going backwards:
*/
if (unlikely(prev > now)) {
__raw_spin_lock(&sync_lock);
max_warp = max(max_warp, prev - now);
nr_warps++;
__raw_spin_unlock(&sync_lock);
}
}
}
/*
* Source CPU calls into this - it waits for the freshly booted
* target CPU to arrive and then starts the measurement:
*/
void __cpuinit check_tsc_sync_source(int cpu)
{
int cpus = 2;
/*
* No need to check if we already know that the TSC is not
* synchronized:
*/
if (unsynchronized_tsc())
return;
printk(KERN_INFO "checking TSC synchronization [CPU#%d -> CPU#%d]:",
smp_processor_id(), cpu);
/*
* Reset it - in case this is a second bootup:
*/
atomic_set(&stop_count, 0);
/*
* Wait for the target to arrive:
*/
while (atomic_read(&start_count) != cpus-1)
cpu_relax();
/*
* Trigger the target to continue into the measurement too:
*/
atomic_inc(&start_count);
check_tsc_warp();
while (atomic_read(&stop_count) != cpus-1)
cpu_relax();
/*
* Reset it - just in case we boot another CPU later:
*/
atomic_set(&start_count, 0);
if (nr_warps) {
printk("\n");
printk(KERN_WARNING "Measured %Ld cycles TSC warp between CPUs,"
" turning off TSC clock.\n", max_warp);
mark_tsc_unstable("check_tsc_sync_source failed");
nr_warps = 0;
max_warp = 0;
last_tsc = 0;
} else {
printk(" passed.\n");
}
/*
* Let the target continue with the bootup:
*/
atomic_inc(&stop_count);
}
/*
* Freshly booted CPUs call into this:
*/
void __cpuinit check_tsc_sync_target(void)
{
int cpus = 2;
if (unsynchronized_tsc())
return;
/*
* Register this CPU's participation and wait for the
* source CPU to start the measurement:
*/
atomic_inc(&start_count);
while (atomic_read(&start_count) != cpus)
cpu_relax();
check_tsc_warp();
/*
* Ok, we are done:
*/
atomic_inc(&stop_count);
/*
* Wait for the source CPU to print stuff:
*/
while (atomic_read(&stop_count) != cpus)
cpu_relax();
}
#undef NR_LOOPS